BACKGROUND INFORMATION
Warping machines and sizing machines are known as machines for preparing yarns for weaving. A warping machine is adapted to systematically arrange a number of yarns which are drawn out from a number of packages held by creels and wind the same on a warper's beam or drum. A sizing machine is adapted to size warps which are drawn out from a number of warper's beams, dry and wind the same on a sizing beam.
In general, natural or blended yarns are prepared for a weaving process through a warping process and a sizing process. Synthetic yarns may be directly brought into the weaving process through the warping process with no sizing.
The warping operation is adapted to arrange a number of warps in parallel with each other for making a woven fabric, and hence it is most important to maintain a uniform tension of the yarns which are wound on the warper's beam or drum. To this end, various types of tension controllers have been recently proposed for maintaining a constant tension of the yarns in the warping operation.
Japanese Patent Publication (Laying-Open No. 62-238838, Application No. 62-82188) with a claim of priority based on Swiss patent application No. 1286/86-8, discloses a technique which is of interest to the present invention. Japanese Patent Publication No. 62-238838 also corresponds to U.S. Pat. No. 4,819,310. FIG. 1 of the above publication illustrates a tension controller for a drum warper. A number of yarns 103 are drawn out from a number of bobbins 102, which are held by a creel 101, and wound on a warper's drum 107 by a brake force supplier 104, a measuring roller 105 and a deflection roller 106. The measuring roller 105 measures the total tension of the number of yarns 103 passing through the same. The brake force supplier 104 supplies the running yarns 103 with a running resistance, i.e., brake force, thereby increasing the yarn tension. In order to maintain a constant tension of the running yarns 103, the brake control input of the brake force supplier 104 is adjusted in response to the actual yarn tension which is detected by the measuring roller 105. The yarns 103 are driven to run at a constant speed.
Even if the brake control input of the brake force supplier 104 is zero, the running yarns 103 are subjected to tension, which is caused by the release resistance in relation to the bobbins 102 and the air resistance against the running yarn.
The inventor has examined the relationship between changes of a package diameter and the yarn tension. FIG. 2 shows on the ordinate the yarn tension (g) or changes thereof caused when cotton yarns of count No. 40 were drawn out from parallel packages at a speed of 800 m/min, as a function of the package diameter (mm) plotted on the abscissa. As understood from FIG. 2, the yarn tension is lowered with as the package diameter is reduced in the first half where the package diameter is large. In the second half of FIG. 2 where the package diameter is small, on the other hand, the yarn tension is increased as the package diameter is further reduced to reach the maximum value immediately before the packages are used up.
Due to the relation between the package diameter and yarn tension shown in FIG. 2, it is impossible to maintain a constant yarn tension from beginning to end of the warping operation, by using the tension controller disclosed in the above mentioned Japanese Patent Publication No. 62-238838. It is assumed here that the yarn tension is set at 20 g in FIG. 2, for example. In that case, the brake force supplier 104 supplies the yarns with tension T.sub.1, which is evaluated by subtracting yarn tension T.sub.0 in the case of a zero brake control input from the set tension (20 g). The brake force supplier 104 is adapted to apply tension to the running yarns, but is not able to reduce the yarn tension which is caused by a release resistance in relation to the package diameters and air resistance against the running yarn. Referring to FIG. 2, therefore, it is possible to control the actual yarn tension to the set tension to conform by adjusting the brake control input by the brake force supplier 104 until the package diameter is about 45 mm. When the package diameter falls below about 45 mm, however, the actual yarn tension is inevitably increased beyond the set tension even if the brake control input by the brake force supplier 104 is zero.
With reference to FIG. 2, the set tension may be so selected as to maximize the yarn tension T.sub.0 which is caused by the release resistance in relation to the packages and by air resistance against the running yarn, i.e., at about 23 g. In order to select the set tension at such a value, however, it is necessary to considerably increase the brake control input of the brake force supplier 104. In other words, excessive tension T.sub.2 is regularly applied to the running yarns. Hence, the yarns are weakened and frequently breakage or disconnection of the yarn occurs.
Thus, it is extremely difficult to maintain a constant yarn tension from beginning to end of the warping operation by the tension controller disclosed in Japanese Patent Publication No. 62-238838, i.e., a tension controller which adjusts the tension applied to the yarns only by the brake control input to the brake force supplier.